Modern integrated circuits (ICs) typically employ differential signaling. In differential signaling, a signal is conveyed between two separate, active conductors instead of one active conductor and ground. The magnitude of the differential signal is the difference between the two signals rather than the voltages between the two individual signals and ground.
To transmit or receive differential signals to or from an IC, the differential signals are transmitted over a pair of conductors of a lead frame. Referring now to FIGS. 1-2, exemplary packages 10 and 10′ are shown to include an integrated circuit die 12 having pads 14 that are connected by bondwires 16 to leads 20 of a lead frame 24. The packages 10 and 10′ are typically encased in a suitable protective material 24.
Referring now to FIG. 3, differential signal pairs 50-1, 50-2 and 50-3 are connected by leads 20-1A and 20-1B, 20-2A and 20-2B, and 20-3A and 20-3B and bondwires 16-1A and 16-1B, 16-2A and 16-2B, and 16-3A and 16-3B, respectively, to pads 14-1A and 14-1B, 14-2A and 14-2B, and 14-3A and 14-3B, respectively, of the integrated circuit die 12. In each differential pair in FIG. 3, the notation A represents a first polarity conductor and the notation B represents a second polarity conductor, respectively.
Adjacent differential pairs are typically located in close proximity to one another. In some circumstances, the differential pairs may be situated with a first polarity conductor of one pair located immediately adjacent to a second polarity conductor of the same pair on one side and with a second polarity conductor of another pair on the opposite side. Pair coupling tends to occur between adjacent high speed differential signals. For example, pair coupling tends to occur at 60 and 62 in FIG. 3. The magnitude of the difference between the signals carried on the adjacent pairs tends to cause data errors and/or to reduce design margins to an unacceptable level particularly for high speed signals such as Gigabit per second and higher data rates. The pair coupling problem tends to be even greater for low cost plastic lead frames as compared to ball grid array (BGA) packaging with a ground plane shield.